John Davis

@KenG none of those are what is commonly understood as the cosmological principle. In this context the CP is simply understood as spatial homogeneity and isotropy on the very largest scales.

The cosmological principle can be empirically justified from our observations of the observable universe, for the most part we assume it applies beyond the observable universe (after all our observable universe is a mere accident of our position, so there is no obvious reason to assume outside of it is any different). However models such as eternal inflation drop this assumption on scales very much larger than the observable universe.